The present invention relates to a piezoelectric device used for an ink jet printing head. Particularly, this-invention relates to a technique for preventing impurities from diffusing in a lower electrode or a piezoelectric film upon baking the piezoelectric film.
An ink jet printing head used for an ink jet printer, which obtains letters or desired images by discharging ink drops onto printing paper selectively in accordance with print data to be input, comprises a piezoelectric device which functions as a drive source for discharging ink. The piezoelectric device comprises a piezoelectric film held between an upper electrode and a lower electrode.
A silicon dioxide film which is superior mainly as a diaphragm is used as a base plane for forming the piezoelectric device. Since the silicon dioxide film has bad adhesion with a metal of a lower electrode for the piezoelectric device, an adhesive layer made of titanium is often employed between the diaphragm and the lower electrode.
Platinum is often used for the lower electrode. When the piezoelectric film is formed over a non-oriented film in an amorphous state such as silicon oxide, orientation of the piezoelectric film is disturbed, thereby impairing ferroelectricity. With this regard, it is a characteristic of platinum that it becomes an oriented film even if it is formed over a non-oriented film. Accordingly, the piezoelectric film is made oriented by using platinum for the lower electrode and by composing the piezoelectric film to be formed over the lower electrode.
As metals, other than platinum, used for the lower electrode, for example, U.S. Pat. No. 5,122,923, U.S. Pat. No. 5,191,510, an official gazette of Japanese Laid-Open (Kokai) Publication No. Hei 7-245236, and an official gazette of Japanese Laid-Open (Kokai) Publication No. Hei 7-24523 disclose an alloy of platinum and iridium, iridium, and iridium oxide. These metals are used mainly for the purpose of enhancing the properties of dielectrics and preventing any deterioration with age.
However, methods of using the above-described conventional materials cause various inconveniences.
For example, in the lower electrode, titanium composing the adhesive layer diffuses in, and forms an alloy with, platinum composing the lower electrode upon baking a PZT-type piezoelectric precursor film, thereby lowering the piezoelectric properties.
FIG. 10 shows a diffusion ratio of titanium, which is a principal component of the adhesive layer, in platinum which is a principal component of the lower electrode, upon baking the piezoelectric precursor film. The horizontal axis indicates lengths of the lower electrode and the adhesive layer in a film thickness direction, and the vertical axis indicates respective contents of platinum and titanium in percentage. Measurement was conducted by setting the thickness of the lower electrode as 500 nm and the thickness of the adhesive layer as 20 nm. About 30% titanium is mixed in the lower electrode in the vicinity of an interface between the lower electrode and the adhesive layer. About 5% titanium is mixed at an interface between the lower electrode and the piezoelectric film, which is 500 nm apart from the interface between the lower electrode and the adhesive layer. It can be confirmed that, in the lower electrode, platinum and titanium are alloyed unevenly with respect to the film thickness direction and the orientation of columnar crystals of platinum is disturbed. Such a phenomenon causes disturbance of orientation of the piezoelectric film formed over the lower electrode, thereby giving rise to problems such as lowering of the piezoelectric properties of the piezoelectric device. Moreover, as titanium is mixed unevenly in platinum, physical properties of the lower electrode in the thickness direction change. Such a lower electrode makes designing and processing difficult.
Concerning the piezoelectric film, thermal treatment at the time of baking causes titanium to diffuse in the piezoelectric film through the lower electrode, and the existence of titanium in the piezoelectric film deteriorates the piezoelectric properties of the piezoelectric device. This is because the existence of titanium disturbs a stoichiometric ratio in a composition of the piezoelectric film and a layer of low dielectric constant is generated at an interface between the lower electrode and the piezoelectric film.
On the other hand, it is also indicated that when iridium is used for the lower electrode, a leak current considerably increases (as in xe2x80x9cClarification of Leak Mechanism of PZT Capacitor Accumulated on Ir Electrodexe2x80x9d from lecture proceedings for the 59th Applied Physics Society Symposium (published on Sep. 15, 1998), p. 450).
In consideration of various problems described above, it is a first object of this invention to make it possible to prevent the diffusion of impurities in the piezoelectric film or the lower electrode at the time of manufacture, and to provide a piezoelectric device of high reliability, which will not peel off, and a method for manufacturing such a piezoelectric device.
It is a second object of this invention to provide an ink jet printing head which uses such a piezoelectric device, and to provide a method for manufacturing such an ink jet printing head.
It is a third object of this invention to provide a printer which uses such an ink jet printing head.
The present invention is a piezoelectric device exhibiting an electromechanical transducing action, characterized in that it comprises:
an adhesive layer composed of a material which enhances adhesion between a base plane of the piezoelectric device and a lower electrode of the piezoelectric device;
a diffusion preventing layer composed of a material which prevents a phenomenon of diffusion of a component of the adhesive layer into the electrode;
a lower electrode formed over the diffusion preventing layer;
a piezoelectric film which is formed over the lower electrode and which exhibits the electromechanical transducing action; and
an upper electrode formed over the piezoelectric film so as to be paired with the lower electrode.
This invention is a piezoelectric device exhibiting an electromechanical transducing action, characterized in that it comprises:
an adhesive layer composed of a material which enhances adhesion between a base plane of the piezoelectric device and a lower electrode of the piezoelectric device;
a lower electrode formed over the adhesive layer;
a diffusion preventing layer formed over the lower electrode and composed of a material which prevents a phenomenon of diffusion of a component of the adhesive layer into the piezoelectric film and also prevents a phenomenon of diffusion of a component of the piezoelectric film into the lower electrode;
a piezoelectric film which is formed over the diffusion preventing layer and which exhibits the electromechanical transducing action; and
an upper electrode formed over the piezoelectric film so as to be paired with the lower electrode.
The diffusion preventing layer is an alloy of titanium and one device selected from a group consisting of iridium, palladium, rhodium, ruthenium, and osmium.
The adhesive layer contains, as its principal component, one device selected from a group consisting of titanium and chromium.
The lower electrode is composed of platinum as its principal component.
This invention is an ink jet printing head comprising the piezoelectric device of this invention, as a piezoelectric actuator, over a diaphragm which serves as the base plane.
The diaphragm has a laminated structure of silicon dioxide and zirconium oxide.
This invention is a printer comprising the ink jet printing head of this invention as printing means.
This invention is a method for manufacturing a piezoelectric device exhibiting an electromechanical transducing action, characterized in that it comprises:
an adhesive layer forming step of forming, over a base plane of the piezoelectric device, an adhesive layer with a material which enhances adhesion between the base plane and a lower electrode of the piezoelectric device;
a diffusion preventing layer forming step of forming, over the adhesive layer, a diffusion preventing layer with a material which prevents a phenomenon of diffusion of a component of the adhesive layer into the electrode;
a lower electrode forming step of forming, over the diffusion preventing layer, a lower electrode by using a conductive material;
a piezoelectric film forming step of forming, over the lower electrode, a piezoelectric film by using a piezoelectric material; and
an upper electrode forming step of forming, over the piezoelectric film, an upper electrode by using a conductive material.
This invention is a method for manufacturing a piezoelectric device exhibiting an electromechanical transducing action, characterized in that it comprises:
an adhesive layer forming step of forming, over a base plane of the piezoelectric device, an adhesive layer with a material which enhances adhesion between the base plane and a lower electrode of the piezoelectric device;
a lower electrode forming step of forming, over the adhesive layer, a lower electrode by using a conductive material;
a diffusion preventing layer forming step of forming, over the lower electrode, a diffusion preventing layer with a material which prevents a phenomenon of diffusion of a component of the adhesive layer into the piezoelectric film, and which also prevents a phenomenon of diffusion of a component of the piezoelectric film into the lower electrode;
a piezoelectric film forming step of forming, over the diffusion preventing layer, a piezoelectric film by using a piezoelectric material; and
an upper electrode forming step of forming, over the piezoelectric film, an upper electrode by using a conductive material.
The diffusion preventing layer forming step is the step of forming the diffusion preventing layer by using, as its principal component, one device selected from a group consisting of iridium, palladium, rhodium, ruthenium, and osmium as starting materials.
The adhesive layer forming step is the step of forming the adhesive layer by using, as its principal component, one device selected from a group consisting of titanium and chromium.
The lower electrode forming step is the step of forming the lower electrode by using platinum as its material.
The piezoelectric film forming step is the step including a thermal treatment step.